See Robin's paper on this:
http://hanson.gmu.edu/hardstep.pdf
If a step is extremely hard (and thus astronomically unlikely to occur in the lifespan of a planet) then we should expect to see it taking a length of time comparable to typical planetary lifespans divided by the number of steps.
The last two steps occurred suspiciously quickly to be super-hard, and the independent clusters of mammalian braininess in cetaceans and primates [EDIT: (and birds, to a lesser extent)] make the third step questionable too.
Super-difficult life and nervous systems look very plausible to me. A special difficulty involved in the formation of mammals (a feature that predisposed to the later development of intelligent hominids) seems less plausible but not very implausible.
Primates existed for about 85mln years, Cetaceans for about 48mln years, in all this time nothing got even close to Homo except for Homo. (Evolution of cetaceans Evolution of primates) Of course plenty of other large social animals had an opportunity to develop something like Homo for at least 300mln years before that. (Evolution of Reptiles), or more if we seriously treat possibility of intelligent life in the sea.
So life, animals with nervous system (these are linked, as without nervous system animal complexity is very small), and Homo seem safe.
The case...
We have a sample of one modern human civilization, but there are some hints on how likely it was to happen.
Major types of hints are:
Data for:
Data against:
To me it looks like life, animals with nervous systems, Upper Paleolithic-style Homo, language, and behavioral modernity were all extremely unlikely events (notice how far ago they are - vaguely ~3.5bln, ~600mln, ~3mln, ~200k or ~600k, ~50k years ago) - except perhaps language and behavioral modernity might have been linked with each other, if language was relatively late (Homo sapiens only) and behavioral modernity more gradual (and its apparent suddenness is an artifact). Once we have behavioral modernity, modern civilization seems almost inevitable. Your interpretation might vary of course, but at least now you have a lot of data to argue for your position, in convenient format.